10-well delta pattern for secondary recovery



April 30, 1968 D. L. HOYT ET AL 3,380,523

lO-WELL DELTA PATTERN FOR SECONDARY RECOVERY Filed June 28, 1966 2 Sheets-Sheet 1 D. L. HOYT ET AL April 30, 1968 l0-WELL DELTA PATTERN FOR SECONDARY RECOVERY Filed June 28, 1966 2 Sheets-Sheet 2 United States Patent 3,380,523 IO-WELL DELTA PATTERN FOR SECONDARY RECOVERY Donald L. Hoyt and Anthony F. Altamira, Houston, Tex.,

assignors to Texaco Inc., New York, N.Y., a corporation of Delaware Filed June 28, 1966, Ser. No. 561,080 8 Claims. (Cl. 166-9) This invention relates generally to the production of hydrocarbons from underground hydrocarbon-bearing formations, and more particularly, to a method for increasing the overall production of hydrocarbons therefrom.

In exploiting underground hydrocarbon-bearing formations through a plurality of wells, it has been the general practice that when a production well yields an excessive amount of an extraneous fluid other than the hydrocarbons, e.g., water or gas, that production well is shutin and the production of hydrocarbons is started and carried out at other production wells in the field. It is known that in such instances, a substantial amount of hydrocarbons is left behind in the hydrocarbon-bearing formation since such is not considered primarily recoverable economically.

Secondary recovery operations are now an essential part of the over-all program planning for virtually every oil and gas-condensate reservoir in underground hydrocarbon-bearing formations. Some of the operations developed include gas repressuring and water, fire, steam and solvent flooding. Usually, they all employ some geometric patern of injection and production wells, with injection of fluid into some of the wells to displace hydrocarbons in the reservoir toward the production wells.

The front, or interface, between the injection and inplace fluids moves from injection toward production wells, changing shape as it progresses. Due to the pressure sinks around the production wells, a portion of the interface tends to accelerate and cusp into the production wells. Breakthrough of the injected fluid occurs when the interface reaches the production wells. The percentage of the entire reservoir which has been invaded by the injected fluid at that time is referred to as the sweep efficiency or the particular geometric pattern used.

The most commonly used well pattern in secondary recovery is the S-spot pattern, with four injection wells at the corners of a square and a production well at the center. In a production field of S-spot patterns, there are as many injection wells as production wells. Sweep efliciency for the 5-spot pattern is about 71%. Other basic flood patterns sometimes used are the 7-spot, direct line and staggered line drives, with sweep etficiencies of 74, 57 and 78 percent respectively.

In field practice, injection usually will be continued well past breakthrough until the reservoir cannot be produced economically. In this way, some additional sweepout can be achieved, but often there will be large volumes of produced injection fluid to be handled, treated and reinjected. If sweepout prior to breakthrough in a pattern flood were improved, it is very likely that the ultimate recovery would be higher. The time and cost of the operation, to achieve comparable recoveries, would be reduced accordingly.

Accordingly, it is an object of the present invention to provide an improved method for the production and recovery of hydrocarbons, particularly liquid petroleum, from underground hydrocarbon-bearing formations.

Another object of this invention is to provide a method whereby the areal sweep efficiency in pattern flooding is improved.

These and other objects, advantages and features of this invention will become apparent from a consideration of the specification with reference to the figures of the accompanying drawings wherein:

FIG. 1 discloses a conventional S-spot pattern in a field undergoing secondary recovery illustrating the interface of the injected fluid at breakthrough at the production wells.

FIG. 2 discloses the formation of 10-well delta pattern in a field of staggered equidistantly spaced wells; and

FIGS. 3 and 4 illustrate the movements of the interface of the injected fluid during the two phases of the exploitation plan in one l0 well delta pattern unit in a field undergoing secondary recovery.

In our copending, coassigned application for patent Ser. No. 517,052, filed Dec. 28, 1965, for Interface Advance Control in Pattern Floods by Use of Control Wells, there is disclosed how an increased amount of hydrocarbons is produced and recovered from an underground hydrocarbon-bearing formation by employing at least three wells, penetrating such a formation, which wells can be in-line, to produce hydrocarbons from the formation via two of these wells including the middle well, as disclosed in the coassigned U.S. Patent No. 3,109,487, issued to Donald L. Hoyt, on Nov. 5, 1963.

It is understood that the failure of the driving flood in secondary recovery operations to contact or sweep all the hydrocarbon area is due to the development, in the interface, of a cusp which advances toward the production well. If other portions of the interface could be made to keep up, or if the cusp formation were delayed, complete areal sweep would be possible. In the above cited coassigned application, a production control well is positioned between the injection well and the production well and is kept on production even after the injection fluid reaches it. In this manner, the cusp is pinned down at the control well and while the area swept out by the injection fluid before breakthrough at the last production well is increased, there is an unwanted handling of considerable quantities of injection fluid at the control well.

Another aspect to increase the sweepout efficiency is disclosed in our copending, coassigned application for patent Ser. No. 516,891, filed Dec. 28, 1965, for Interface Advance Control in Pattern Floods by Retarding Cusp Formation, and involves the retardation of the development of the cusp toward the production well. The method of achieving more uniform advance is to control the flow gradients so that the interface is spread out. This can be done either by choosing a particular geometry of well positions or by adjusting the relative production rates so that the velocity of advance is not predominantly in one direction. It can be done also by shifting the gradients frequently, in both direction and magnitude, thus preventing any one section of an interface from advancing too far out of line.

The figures of the drawings schematically disclose and illustrate the practice and the advantages of our invention with well pattern and areal sweepout examples which are obtainable and have been observed both in secondary recovery operations and in potentiometric model studies which simulate secondary recovery operations. The model studies indicate a sweepout obtained in an ideal reservoir, although the recovery from an actual sweepout of a particular field may be greater or less, depending on field parameters. The procedures to be described are based on the following set of experimental conditions and assumptions:

(1) All the units in any pattern :are balanced and produce at the same rate. This requires that wells on the edges of a pattern unit produce or inject at or of the rate of interior wells, depending inversely on the number of pattern units with which they are associated;

(2) The total amount of fluid injected must be equal a) to the fiuid produced for each pattern unit, as well as for the whole pattern;

(3) The mobility ratio of the displacing placed tiuid is 1.0;

(4) The permeability and sand thickness mation is uniform; and

(5) Gravitational effects are not considered.

Throughout the drawings, the same symbolic indicators will be maintained as follows: P P and P represent respectively Wells at the corners, along the sides, and the interior or center wells; and, a solid circle indicates a production well, a crossed circle indicates a shut-in well, and an arrowed circle indicates an injection well.

FIG. 1 illustrates the interface of the injected fluid at breakthrough at the production well of a single conventional 5-spot pattern unit in a production field undergoing secondary recovery, wherein the corner wells of each pattern unit are injection wells, while the inner center well is used for production. Such a procedure will produce a sweepout of approximately 71%. In this pattern, the ratio of the distance between the rows of wells to the distmce between the in-line wells is 0.5, i.e. d/a= In our concurrently filed, coassigned application for patent Ser. No. 561,110 for 7-well Delta Pattern for Secondary Recovery, there is disclosed the formation of the 7-well delta pattern from three rows of wells in a staggered line field. By extending the pattern to four rows, a 10-well delta pattern is formed, so called because of its triangular shape and the ten wells which are in the basic unit.

Referring to FIG. 2, there is disclosed the formation of a l0well delta pattern comprising nine wells each spaced equi-distnntly from each other, at the vertices of an equilateral triangle and intermediate these wells on the sides of this triangle plus a single interior or center well. The two Wells on each of the sides, the wells at each of the vertices and in the center of this triangle or delta pattern are part of an original staggered equidistant well spacing, wherein the ratio of the distance between the rows of wells to the distance between the inline wells is 0.866, i.e. d/cr=0.866, so that no additional drilling is necessary to complete the pattern.

FIGS. 3 and 4 disclose the consecutive steps of the two phase, secondary recovery exploitation plan as would occur in one 10-well delta pattern unit.

FIGS. 3 discloses the first phase of the interface formation wherein driving fluid is injected into the formation through the center well (P with production from the equidistantly spaced wells (P at the corners or the vertices and the side wells (P of an equilateral triangle until breakthrough of the injected driving fluid occurs at the side wells. At this breakthrough, a cam-like hexamerous area defined by the cusp formations ending at the production side wells has been swept out, with the area remaining to be swept indicated by cross-hatching in FIG. 3. In this phase of the plan, the ratio of the injection well rate to the production well rate is 3 to 1, since there are seven production wells for each two injection wells in this pattern throughout the production field. Both injection and production rates are uniformly distributed.

FIG. 4 discloses the second phase of the exploitation plan, wherein the side wells (P are shut in, injection is continued through the center well (P and production continued through the corner wells (P until breakthrough of the injected fluid thereat, the symbolic indicators being used as noted previously. The sweepout attained at breakthrough of the injected fluid at the corner production wells exceeds 86% sweep efficiency. The ratio of the injection well rate to the production well rate is 1 to 2 since there is one production well for each two injection wells in this pattern throughout the production field, with the injection and production rates being uniformly distributed.

to the disof the for- The unswept area at the end of the second phase as disclosed in FIG. 4 extends along the three sides of the pattern where production wells are available, and thus are regions of pressure sinks. Also, the region around the side wells (P has been resaturated with formation fluid permitting the resumption of production at the side wells (P thereby allowing additional sweepout before breakthrough of injected fluid at these wells. Therefore, most of the unswept areas probably could be swept if production were continued past breakthrough, a common field practice. Again, this is a significant advantage since in many patterns much of the unswept area is far from the production well and in the region of such low flow gradient as to be effectively inaccessible.

Thus, further production may be continued after breakthrough with comparatively less production of the injected fluid than in the ordinary production following breakthrough.

it is recognized that the more than 86% sweepout obtained by the use of the 10-well delta pattern exploited as disclosed herein is an idealization, as in the case for the 71% for the conventional five-spot pattern. None of these values is likely to be achieved in the field, but the relative superiority of the sweepout which may be obtained by the use of the 10-well delta pattern is clear and is independent of inhomogeneities.

The basic technique employed to increase sweep efficiency controls the advance of the front in the pattern to achieve large areal covenage, by shifting and adjusting the geometric position, direction and magnitude of the flow of pressure gradients. If the positions of the wells in the pattern are favorable, by changing the wells used for injection and production, high flow gradients can be made to cover most of the pattern. The pattern and procedure disclosed herein has a high sweep efilciency, leaving the unswept areas at breakthrough in the regions of strong flow gradients adjacent the production wells, and thus readily swept by production past breakthrough.

It is apparent, however that without departing from the principles of the disclosure, minor variations can be made in the exploitation plan described herein which will yield as good a sweepout. It is not necessary, for example, that production rates be uniform in the first phase of the procedure. Thus, a production well rate on the corner wells (P slightly less than the production well rate on the side wells (P would hold back the extended curved portions of the interface during the first phase such that an even greater sweepout than 86% may be realized at breakthrough at the corner wells (P at the end of the second phase.

The sweepout and volumes of injected fluid produced vary from one pattern to another, and also appear to be functions of rate distribtuion and distance parameters within a given pattern.

In evaluating the preformance of a flood pattern, or in comparing performances of different patterns, there are three main considerations:

(a) Percentage of sweep (b) Volume of injection fluid handled (c) Time to achieve the sweep.

For a given total field production rate, it will not be possible generally to obtain an increase in sweepont without at least a proportionate increase of time, which is to be expected. However, if the extra time involved is disproportionately long, the gain in sweepout may not be economically worthwhile.

Any pattern and/ or rate distribution which retards the development, or the advance, of a cusp towards the production wells will increase the sweepout of a field. As mentioned previously, two principal means of doing this are: (a) pinning down the cusp by locating production wells between the injection source and the outer production well, and keeping these inner (or control) wells on production after breakthrough, and (b) spreading out the cusp by pulling the front toward side wells until breakthrough thereat before allowing it to proceed toward the corner production wells of a pattern unit, as disclosed in our above cited coassigned applicatrons.

The spreading out of the cusp is in general a more advantageous procedure. It yields as good or better sweepout with less production of injection fluid. Further, a higher rate distribution on corner wells of pattern units generally results in much less overall production of injection fluid, but also in less sweep (although exceptions can be found in more complicated patterns).

The advantages of the methods disclosed above are evident. Fewer injection wells are required, more reservoir fluid is recovered prior to breakthrough of injection fluid, and so more ultimate recovery is obtained, as compared with other methods generally employed in secondary recovery operation.

Although emphasis has been placed in this disclosure on the practice of this invention as directed to a secondary recovery operation, particularly employing water or other similar aqueous fluid as the injection fluid or displacement fluid, the advantages obtainable in the practice of this invention are also realized in primary hydrocarbon production operations wherein the hydrocarbonbearing formation is under the influence of a water drive or gas drive, or both a water and a gas drive, and also in the instance of a secondary recovery operation wherein a gas, such as natural gas, is employed as the injunction fluid.

As will be apparent to those skilled in the art, in the light of the accompanying disclosure, other changes and alternatives are possible in the practice of this invention without departing from the spirit or scope thereof.

We claim:

1. A method of producing hydrocarbons from an underground hydrocarbon-bearing formation involving wells located at the vertices and on the sides of a triangle and a well located therewithin which comprises introducing fluid into said formation via the well located within said triangle, and producing hydrocarbons from said formation via the wells located at said vertices and on said sides of said triangle until breakthrough at the wells on said sides of said triangle of the fluid introduced into said formation, and thereupon ceasing producing hydrocarbons from said wells on said sides of said triangle while still producing hydrocarbons from the Wells located at said vertices of said triangle and introducing fluid into said formation through said well within said triangle.

2. In the method of producing hydrocarbons as defined in claim 1, the additional steps of ceasing producing hydrocarbons from said wells located at said vertices upon breakthrough of the fluid introduced into said formation,

and thereafter again initiating producing hydrocarbons from said wells on said sides of said triangle.

3. In the method of producing hydrocarbons as defined in claim 1, the rates of introducing fluid into said formation and producing hydrocarbons from said formation being controlled so that the breakthrough at the wells on said sides of said triangle of the fluid introduced into said formation occurs simultaneously.

4. In the method of producing hydrocarbons as defined in claim 3, the rates of producing hydrocarbons from said wells located at said vertices and on said sides being controlled to modify the formation of the interface between adjacent side wells.

5. In the method of producing hydrocarbons as defined in claim 1, said wells located at the vertices of said triangle defining an equilateral structure, and said well located therewithin being positioned equidistantly from said vertices.

6. In the method of producing hydrocarbons as defined in claim 5, said wells located on said sides and at said vertices of said triangle being spaced equally from each other along the periphery thereof, said wells at said vertices and on said sides being spaced equidistantly from each other on each of said sides.

7. In the method of producing hydrocarbons as defined in claim 6, said wells on each of said sides of said triangle being at least two in number and spaced equidistantly from each other, the well within said triangle being located equidistantly from the vertices of said triangle, said well at said vertices of and on said sides of and within said triangle being at least ten in number.

8. In the method of producing hydrocarbons as defined in claim 7, the wells located at said vertices and on said sides of said triangle and therewithin being spaced apart and aligned in rows to define a pattern unit in a production field wherein the ratio of the distance between the rows of wells to the distance between the in-line wells is 0.866.

References Cited UNITED STATES PATENTS 2,885,002 5/1959 Jenks 166-9 3,113,616 12/1963 Dew et al 166-9 3,113,617 12/1963 Oakes 166-9 3,113,618 12/1963 Oakes 166-9 3,120,870 2/1964 Santourian 166-9 3,143,169 8/1964 Foulks 166-9 3,205,943 9/1965 Foulks 166-9 CHARLES E. OCONNELL, Primary Examiner. DAVID H. BROWN, Assistant Examiner. 

1. A METHOD OF PRODUCING HYDROCARBON FROM AN UNDERGROUND HYDROCARBON-BEARING FORMATION INVOLVING WELLS LOCATED AT THE VERTICES AND ON THE SIDES OF A TRIANGLE AND A WELL LOCATED THEREWITHIN WHICH COMPRISES INTRODUCING FLUID INTO SAID FORMATION VIA THE WELL LOCATED WITHIN SAID TRIANGLE, AND PRODUCING HYDROCARBON FROM SAID FORMATION VIA THE WELLS LOCATED AT SAID VERTICES AND ON SAID SIDES OF SAID TRIANGLE UNTIL BREAKTHROUGH AT THE WELLS ON SAID SIDES OF SAID TRIANGLE OF THE FLUID INTRODUCED INTO SAID FORMATION, AND THEREUPON CEASING PRODUCING HYDROCARBONS FROM SAID WELLS ON SAID SIDES OF SAID TRIANGLE WHILE STILL PRODUCING HYDROCARBONS FROM THE WELLS LOCATED AT SAID VERTICES OF SAID TRIANGLE AND INTRODUCING FLUID INTO SAID FORMATION THROUGH SAID WELL WITHIN SAID TRIANGLE. 